This invention relates to a circuit to prevent excessive discharge of a rechargeable battery which is detachable or permanently housed within an electrical apparatus.
If a rechargeable battery (which can be used repeatedly by recharging) is discharged to a point where the battery voltage drops below a given voltage, its performance will be degraded and battery lifetime will in turn be significantly shortened. To prevent excessive discharge of a rechargeable battery, methods have been employed that detect battery voltage and cut-off battery discharge when that voltage drops below a given value. For example, according to Japanese Patent Disclosure No. 4-33271, issued Feb. 4, 1992, a switching device is connected in series with the rechargeable battery and that switching device is turned off to cut-off battery discharge when battery voltage drops below a given value.
In this same disclosure, the switching device is maintained in the off state until the battery reaches a voltage slightly higher than the previously mentioned given value. This system then turns the switching device on to re-establish battery discharge when the slightly higher voltage is reached. In other words, the system has a hysteresis characteristic between the discharge cut-off voltage and the discharge re-start voltage which can be obtained, for example, by using a Schmitt trigger circuit.
The use of hysteresis between the discharge cut-off and re-start voltages described above prevents switching device chattering (intermittent connection due to repeated on-off switching) during self-recovery of the battery voltage after discharge cut-off. However, setting the hysteresis width between the discharge cut-off voltage and the re-start voltage to prevent chattering is difficult.
If the hysteresis width is too wide, rechargeable battery discharge is not possible until the battery is considerably charged. On the other hand, if the hysteresis width is too narrow, switching device chattering cannot prevented.
Further, because of the necessity to detect the re-start voltage after cutting off battery discharge, the discharge control circuit including the Schmitt trigger must remain activated. As a result, discharge control circuit current drain on the rechargeable battery can lead to excessive battery discharge.
It is thus an object of the present invention to reliably prevent excessive rechargeable battery discharge with a simple system.